Background Art
1. Composite Manufacturing
Fiber-reinforced organic resin matrix composites have a high strength-to-weight ratio or a high stiffness-to-weight ratio and desirable fatigue characteristics that make them increasingly popular as a replacement for metal in aerospace applications where weight, strength, or fatigue is critical. Organic resin composites, be they thermoplastics or thermosets, are expensive today. Improved manufacturing processes would reduce touch labor and forming time.
Prepregs combine continuous, woven, or chopped reinforcing fibers with an uncured, matrix resin, and usually comprise fiber sheets with a thin film of the matrix. Sheets of prepreg generally are placed (laid-up) by hand or with fiber placement machines directly upon a tool or die having a forming surface contoured to the desired shape of the completed part or are laid-up in a flat sheet which is then draped and formed over the tool or die to the contour of the tool. Then the resin in the prepreg lay up is consolidated (i.e. pressed to remove any air, gas, or vapor) and cured (i.e., chemically converted to its final form usually through chain-extension) in a vacuum bag process in an autoclave (i.e., a pressure oven) to complete the part.
The tools or dies for composite processing typically are formed to close dimensional tolerances. They are massive, must be heated along with the workpiece, and must be cooled prior to removing the completed part. The delay caused to heat and to cool the mass of the tools adds substantially to the overall time necessary to fabricate each part. These delays are especially significant when the manufacturing run is low rate where the dies need to be changed frequently, often after producing only a few parts of each kind. An autoclave has similar limitations; it is a batch operation.
In hot press forming, the prepreg is laid-up to create a preform, which is bagged (if necessary), and placed between matched metal tools that include forming surfaces to define the internal, external, or both mold lines of the completed part. The tools and composite preform are placed within a press and then the tools, press, and preform are heated.
The tooling in autoclave or hot press fabrication is a significant heat sink that consumes substantial energy. Furthermore, the tooling takes significant time to heat the composite material to its consolidation temperature and, after curing the composite, to cool to a temperature at which it is safe to remove the finished composite part.
As described in U.S. Pat. No. 4,657,717, a flat composite prepreg panel was sandwiched between two metal sheets made from a superplastically formable alloy and then formed against a die having a surface precisely contoured to the final shape of the part.
Attempts have been made to reduce composite fabrication times by actively cooling the tools after forming the composite part. These attempts have shortened the time necessary to produce a composite part, but the cycle time for and cost of heating and cooling remain significant contributors to overall fabrication costs. Designing and making tools to permit their active cooling increases their cost.
Boeing described a process for organic matrix forming and consolidation using induction heating in U.S. Pat. No. 5,530,227. There, prepregs were laid up in a flat sheet and were sandwiched between aluminum susceptor facesheets. The facesheets were susceptible to heating by induction and formed a retort to enclose the prepreg preform. To ensure an inert atmosphere around the composite during curing and to permit withdrawing volatiles and outgassing from around the composite during the consolidation, the facesheets were welded around their periphery. However, such welding unduly impacts the preparation time and the cost for part fabrication. It also ruined the facesheets (i.e., prohibited their reuse which added a significant cost penalty to each part fabricated with this approach). Boeing also described in U.S. Pat. No. 5,599,472 a technique that readily and reliably seals facesheets of the retort without the need for welding and permits reuse of the facesheets in certain circumstances. This "bag-and-seal" technique applies to both resin composite and metal processing.
2. Processing in an Induction Press
The dies or tooling for induction processing are ceramic because a ceramic is not susceptible to induction heating and, preferably, is a thermal insulator (i.e., a relatively poor conductor of heat). Cast ceramic tooling is strengthened and reinforced internally, with fiberglass rods or other appropriate reinforcements and externally with metal or other durable strongbacks to permit it to withstand the temperatures and pressures necessary to form, to consolidate, or otherwise to process the composite materials or metals. Cast ceramic tools cost less to fabricate than metal tools of comparable size and have less thermal mass than metal tooling, because they are unaffected by the induction field. Because the ceramic tooling is not susceptible to induction heating, it is possible to embed induction heating elements in the ceramic tooling and to heat the composite or metal retort without significantly heating the tools. The induction heating elements themselves connect to form a water cooling network. Thus, induction heating can reduce the time required and energy consumed to fabricate a part.
While graphite or boron fibers can be heated directly by induction, most organic matrix composites require a susceptor in or adjacent to the composite material preform to achieve the necessary heating for consolidation or forming. The susceptor is heated inductively and transfers its heat principally through conduction to the preform or workpiece that, in Boeing's prior work, is sealed within the susceptor retort. Enclosed in the metal retort, the workpiece does not experience the oscillating magnetic field which instead is absorbed in the retort sheets. Heating is by conduction from the retort to the workpiece.
Induction focuses heating on the retort (and workpiece) and eliminates wasteful, inefficient heat sinks. Because the ceramic tools in Boeing's induction heating workcell do not heat to as high a temperature as the metal tooling of conventional, prior art presses, problems caused by different coefficients of thermal expansion between the tools and the workpiece are reduced. Furthermore, this process is energy efficient because significantly higher percentages of the input energy go to heating the workpiece than occurs with conventional presses. The reduced thermal mass and ability to focus the heating energy permits the operating temperature to be changed rapidly which improves the products produced by Boeing's workcell. Finally, the shop environment is not heated as significantly from the radiation of the large thermal mass of a conventional press, and is a safer and more pleasant environment for the press operators.
In induction heating for consolidating and/or forming organic matrix composite materials, Boeing has placed a thermoplastic organic matrix composite preform of PEEK or ULTEM, for example, within a metal susceptor envelope (i.e., retort). These thermoplastics have a low concentration of residual volatile solvents and are easy to use. The susceptor facesheets of the retort are inductively heated to heat the preform. A consolidation and forming pressure is applied to consolidate and, if applicable, to form the preform at its curing temperature. The sealed susceptor sheets form a pressure zone. The pressure zone is evacuated in the retort in a manner analogous to conventional vacuum bag processes for resin consolidation or, for low volatiles resins, like ULTEM, this zone can be pressurized to enhance consolidation. The retort is placed in an induction heating press on the forming surfaces of dies having the desired shape of the molded composite part. After the retort (and preform) are inductively heated to the desired elevated temperature, a differential pressure is applied (while maintaining the vacuum in the pressure zone around the preform) across the retort. The retort functions as a diaphragm in the press to form the preform against the die into the desired shape of the completed composite panel.
The retort often includes three susceptor sheets sealed around their periphery to define two pressure zones. The first pressure zone surrounds the composite panel/preform or metal workpiece and is evacuated and maintained under vacuum. The second pressure zone is pressurized (i.e., flooded with gas) at the appropriate time and rate to help form the composite panel or workpiece. The shared wall of the three layer sandwich that defines the two pressure zones acts as a diaphragm in this situation.
Boeing can perform a wide range of manufacturing operations in its induction heating press. These operations have optimum operating temperatures ranging from about 350.degree. F. (175.degree. C.) to about 1950.degree. F. (1066.degree. C.). For each operation, the temperature usually needs to be held relatively constant for several minutes to several hours while the operations are completed. While temperature control can be achieved by controlling the input power fed to the induction coil, Boeing has discovered a better and simpler way that capitalizes on the Curie temperature. By judicious selection of the metal or alloy in the retort's susceptor facesheets, excessive heating can be avoided irrespective of the input power. With improved control and improved temperature uniformity in the workpiece, better products can be produced. Boeing's method capitalizes on the Curie temperature phenomenon to control the absolute temperature of the workpiece and to obtain substantial thermal uniformity in the workpiece, by matching the Curie temperature of the susceptor to the desired temperature of the induction heating operation being performed. This temperature control method is explained in greater detail in Boeing's U.S. Pat. No. 5,728,309 which is incorporated by reference.
3. Thermoplastic Welding
Three major joining technologies exist for aerospace composite structure: mechanical fastening; adhesive bonding; and welding. Both mechanical fastening and adhesive bonding are costly, time consuming assembly steps that introduce excess cost even if the parts that are assembled are fabricated from components produced by an emerging, cost efficient process. Mechanical fastening requires expensive hole locating, drilling, shimming, and fastener installation, while adhesive bonding often requires complicated surface pretreatments.
In contrast, thermoplastic welding, which eliminates fasteners, features the ability to join thermoplastic composite components at high speeds with minimum touch labor and little, if any, pretreatments. In Boeing's experience, the welding interlayer (comprising the susceptor and surrounding thermoplastic resin either coating the susceptor or sandwiching it) also can simultaneously take the place of shims required in mechanical fastening. As such, composite welding holds promise to be an affordable joining process. For "welding " thermoplastic and thermoset composite parts together, the resin that the susceptor melts functions as a hot melt adhesive. If fully realized, the thermoplastic-thermoset bonding will further reduce the cost of composite assembly.
There is a large stake in developing a successful induction welding process. Its advantages versus traditional composite joining methods are:
reduced parts count versus fasteners PA1 minimal surface preparation, in most cases a simple solvent wipe to remove surface contaminants PA1 indefinite shelf life at room temperature PA1 short process cycle time, typically measured in minutes PA1 enhanced joint performance, especially hot/wet and fatigue PA1 permits rapid field repair of composites or other structures.
There is little or no loss of bond strength after prolonged exposure to environmental influences.
U.S. Pat. No. 4,673,450 describes a method to spot weld graphite fiber reinforced PEEK composites using a pair of electrodes After roughening the surfaces of the prefabricated PEEK composites in the region of the bond, Burke placed a PEEK adhesive ply along the bond line, applied a pressure of about 50-100 psi through the electrodes, and heated the embedded graphite fibers by applying a voltage in the range of 20-40 volts at 30-40 amps for approximately 5-10 seconds with the electrodes. Access to both sides of the assembly is required in this process which limits its application.
Prior art disclosing thermoplastic welding with induction heating is illustrated by U.S. Pat. Nos. 3,966,402 and 4,120,712. The metallic susceptors are of a conventional type having a regular pattern of openings of traditional manufacture. Achieving a uniform, controllable temperature in the bondline, which is crucial to preparing a thermoplastic weld of adequate integrity to permit use of welding in aerospace primary structure, is difficult with those conventional susceptors, as was discussed and illustrated in U.S. Pat. No. 5,500,511.
Thermoplastic welding is a process for forming a fusion bond between two faying thermoplastic faces of two parts. A fusion bond is created when the thermoplastic on the surface of the two thermoplastic composite parts is heated to the melting or softening point and the two surfaces are brought into contact, so that the molten thermoplastic mixes, and the surfaces are held in contact while the thermoplastic cools below the softening temperature.
Simple as the thermoplastic welding process sounds, and easy as it is to perform in the laboratory on small pieces, it becomes difficult to perform reliably and repeatably in a real factory on full-scale parts to build a large structure such as an airplane wing box. The difficulty is in getting the proper amount of heat to the bondline without overheating the entire structure. Considerable difficulty can also be encountered in achieving intimate contact of the faying surfaces of the two parts at the bondline during heating and cooling despite the normal imperfections in the flatness of composite parts, thermal expansion of the thermoplastic during heating to the softening or melting temperature, flow of the thermoplastic out of the bondline under pressure (i.e., squeeze out), and then contraction of the thermoplastic in the bondline during cooling. The exponential decay of the strength of magnetic fields dictates that, in induction welding processes, the susceptible structure closest to the induction coil will be the hottest, since it experiences the strongest field. Therefore, it is difficult to obtain adequate heating at the bond line between two graphite or carbon fiber reinforced resin matrix composites relying on the susceptibility of the fibers alone as the source of heating in the assembly. For the inner plies to be hot enough to melt the resin, the outer plies closer to the induction coil and in the stronger magnetic field are too hot. The matrix resin in the entire piece of composite melts. The overheating results in porosity in the product, delamination, and, in some case, destruction or denaturing of the resin. To avoid overheating of the outer plies and to insure adequate heating of the inner plies, Boeing uses a susceptor of significantly higher conductivity than the fibers to peak the heating selectively at the bondline. An electromagnetic induction coil heats a susceptor to melt and cure a thermoplastic resin (also sometimes referred to as an adhesive) to bond the elements of the assembly together.
The current density in the susceptor may be higher at the edges of the susceptor than in the center because of the nonlinearity of the coil, such as occurs when using a cup core induction coil like that described in U.S. Pat. No. 5,313,037. Overheating the edges of the assembly can result in underheating the center, with either condition leading to inferior welds because of non-uniform curing. It is necessary to have an open or mesh pattern in the susceptor embedded at the bondline to allow the resin to create the fusion bond between the composite elements of the assembly when the resin heats and melts.
a. Moving Coil Welding Processes
In U.S. Pat. No. 5,500,511, Boeing described a tailored susceptor for approaching the desired temperature uniformity. This susceptor, designed for use with the cup coil of U.S. Pat. No. 5,313,037, relied upon carefully controlling the geometry of openings in the susceptor (both their orientation and their spacing) to distribute the heat evenly. The use of a regular array of anisotropic, diamond shaped openings with a ratio of the length (L) to the width (W) greater than 1 was suggested to provide a superior weld by producing a more uniform temperature than that obtainable by using a susceptor having a similar array, but one where the L/W ratio was one. By changing the length to width ratio (the aspect ratio) of the diamond-shaped openings in the susceptor, a large difference in the longitudinal and transverse conductivity in the susceptor was achieved, which thereby tailored the current density within the susceptor. A tailored susceptor having openings with a length (L) to width (W) ratio of 2:1 has a longitudinal conductivity about four times the transverse conductivity. In addition to tailoring the shape of the openings to tailor the susceptor, the current density was altered in regions near the edges by increasing the foil density (i.e., the absolute amount of metal). Increasing the foil density along the edge of the susceptor increases the conductivity along the edge and reduces the current density and the edge heating. Foil density was increased by folding the susceptor to form edge strips of double thickness or by compressing openings near the edge of an otherwise uniform susceptor. Boeing found these susceptors difficult to reproduce reliably. Also, their use forced careful placement and alignment to achieve the desired effect.
The tailored susceptor was designed to be used with the cup coil of U.S. Pat. No. 5,313,037, where the magnetic field is strongest near the edges because the central pole creates a null at the center. Therefore, the tailored susceptor was designed to counter the higher field at the edges by accommodating the induced current near the edges. The high longitudinal conductivity encouraged induced currents to flow longitudinally.
Boeing's salvaged susceptor for thermoplastic welding, which is described in U.S. Pat. No. 5,508,496, controls the current density pattern during eddy current heating by an induction coil to provide substantially uniform heating to a composite assembly and to insure the strength and integrity of the weld in the completed part. This susceptor is particularly desirable for welding ribs between prior welded spars using an asymmetric induction coil (described in U.S. Pat. No. 5,444,220, which is incorporated by reference herein), because, with that coil, it provides a controllable area of intense, uniform heating, a trailing region with essentially no heating, and a leading region with minor preheating.
The power (P) [or power density] which the susceptor dissipates as heat follows the well-known equation for power loss in a resistor: P=(J.sup.2)(R) wherein J is the eddy current (or its density) and R is the impedance (i.e., resistance) of any segment of the eddy path. The heating achieved directly corresponds to the power (or power density).
We achieve better performance (i.e., more uniform heating) in rib welding by using a salvaged susceptor having edge strips without openings. The resulting susceptor, then, has a center portion with a regular pattern of opening and solid foil edges, which Boeing refers to as salvage edge strips. The susceptor is embedded in a thermoplastic resin to make a susceptor/resin tape that is easy to handle and to use in preforming the composite pieces prior to welding. Also, it has been discovered that, with a salvaged susceptor, the impedance of the central portion should be anisotropic with a lower transverse impedance than the longitudinal impedance. Here, the L/W ratio of diamond shaped openings should be less than or equal to one. That is, unlike Boeing's tailored susceptor of U.S. Pat. No. 5,500,511, "L" for the salvaged susceptor should be less than "W". With this new salvaged susceptor in the region immediately under the asymmetric induction work coil, current is encouraged to flow across the susceptor to the edges where the current density is lowest and the conductivity, highest.
Generally, Boeing forms the salvaged susceptor somewhat wider than normal so that the salvage edge strips are not in the bondline. The salvage edge strips are removed after forming the weld, leaving only a perforated susceptor foil in the weld. This foil has a relatively high open area fraction.
Significant effort has been expended in developing inductor and susceptor systems to optimize the heating of the bondline in thermoplastic assemblies. Induction coil structures and tailored susceptors have now been developed that provide adequate control and uniformity of heating of the bondline. However, a big hurdle that remains to perfecting the process to the point of practical utility for producing large scale aerospace-quality structures in a production environment is the aspect of the process dealing with the control of the surface contact of the faying surfaces of the two parts to be welded together. Additional important factors are the timing, intensity, and schedule of heat application so the material at the faying surfaces is brought to and maintained within the proper temperature range for the requisite amount of time for an adequate bond to form, and is maintained in intimate contact while the melted or softened material hardens in its bonded condition.
Large scale parts such as wing spars and ribs, and the wing skins that are bonded to the spars and ribs, are typically on the order of 20-30 feet long at present, and potentially can be hundreds of feet in length when the process is perfected for commercial transport aircraft. Parts of this magnitude are difficult to produce with perfect flatness. Instead, the typical part will have various combinations of surface deviations from perfect flatness, including large scale waviness in the direction of the major length dimension, twist about the longitudinal axis, dishing or sagging of "I" beam flanges, and small scale surface defects such as aspirates and depressions. These irregularities interfere with full surface area contact between the faying surfaces of the two parts and actually result in surface contact only at a few "high points" across the intended bondline. Additional surface contact can be achieved by applying pressure to the parts to force the faying surfaces into contact, but full intimate contact is difficult or impossible to achieve in this way. Applying heat to the interface by electrically heating the susceptor in connection with pressure on the parts tends to flatten the irregularities further, but the time needed to achieve full intimate contact with the use of heat and pressure is excessive, can result in deformation of the top part, and tends to raise the overall temperature of the "I" beam flanges to the softening point, so they begin to yield or sag under the application of the pressure needed to achieve a good bond.
Boeing's multipass thermoplastic welding process described in U.S. Pat. No. 5,486,684 enables a moving coil welding process to produce continuous or nearly continuous fusion bonds over the full area of the bondline to yield very high strength welds reliably, repeatably and with consistent quality. This process produces improved low cost, high strength composite assemblies of large scale parts fusion bonded together with consistent quality. It also uses a schedule of heat application that maintains the overall temperature of the structure within the limit in which it retains its high strength, so it requires no internal tooling to support the structure against sagging which otherwise could occur above the high strength temperature limit. The process produces nearly complete bondline area fusion on standard production composite material parts having the usual surface imperfections and deviations from perfect flatness. Furthermore, it eliminates fasteners and the expense of drilling holes, inspecting the holes and the fasteners, inspecting the fasteners after installation, sealing between the parts and around the fastener and the hole, reducing mismatch of materials and eliminating arcing from the fasteners.
In the process, an induction heating work coil is passed multiple times over a bondline while applying pressure in the region of the coil to the components to be welded, and maintaining the pressure until the resin hardens. The resin at the bondline is heated to the softening or melting temperature with each pass of the induction work coil. Pressure is also exerted to flow the softened/melted resin in the bondline and reduce the thickness of the bondline while improving the intimacy of the faying surface contact with each pass to militate for complete continuity of bond. The total time at the softened or melted condition of the thermoplastic in the faying surfaces is sufficient to attain deep interdiffusion of the polymer chains in the materials of the two faying surfaces throughout the entire length and area of the bondline. This produces a bondline of improved strength and integrity in the completed part, but the total time of the faying surfaces at softened temperature is in separate time segments. This allows time for the heat in the interface to dissipate without raising the temperature of the entire structure to the degree at which it loses its strength and begins to sag. In this manner the desired shape and size of the final assembly is maintained.
A structural susceptor allows Boeing to include fiber reinforcement within the weld resin to alleviate residual tensile strain otherwise present in an unreinforced weld. The susceptor includes alternating layers of thin film thermoplastic resin sheets and fiber reinforcement (usually woven fiberglass fiber) sandwiching the conventional metal susceptor that is embedded in the resin. While the number of total plies in this structural susceptor is usually not critical, Boeing prefers to use at least two plies of fiber reinforcement on each side of the susceptor. This structural susceptor is described in greater detail in Boeing's U.S. Pat. No. 5,717,191, which is incorporated by reference.
The structural susceptor permits gap filling between the welded composite laminates. This effectively tailors the thickness (number of plies) in the structural susceptor to fill the gaps, thereby eliminating costly profilometry of the faying surfaces and the inherent associated problem of resin depletion at the faying surfaces caused by machining the surfaces to have complementary contours. Standard manufacturing tolerances produce gaps as large as 0.120 inch, which is too wide to create a quality weld using the conventional susceptors.
Boeing can easily tailor the thickness of the structural susceptor to match the measured gap by scoring through the appropriate number of plies of resin and fiber reinforcement and peeling them off. In doing so, a resin side layer will be on both faying surfaces and this layer should insure better performance from the weld.
b. Fixed Coil Induction Welding
Boeing has also experimented with thermoplastic welding using its induction heating workcell and, of course, discovered that the process differs from the moving coil processes because of the coil design and resulting magnetic field. It is believed that Boeing's fixed coil workcell presents promise for welding at faster cycle times than the moving coil processes because multiple susceptors can be heated simultaneously. The keys to the process, however, are achieving controllable temperatures at the bondline in a reliable and reproducible process that assure quality welds of high bond strength. Boeing's fixed coil induces currents to flow in the susceptor differently from the moving coils and covers a larger area. Nevertheless, processing parameters have been developed that permit welding with Boeing's induction heating workcell using a susceptor at the bondline. These processes are described in Boeing's U.S. Pat. Nos. 5,641,422 and 5,723,849.
Another advantage with the fixed coil process is that welding can occur using the same tooling and processing equipment that is used to consolidate the skin, thereby greatly reducing tooling costs. Finally, the fixed coil heats the entire bondline at one time to eliminate the need for shims or profile matching machining that are currently used with the moving coil. Boeing's fixed coil process controls the temperature and protects against overheating by using "smart" susceptors as a retort or as the bondline susceptor material or both. U.S. Pat. Nos. 5,808,281 or 5,728,309.
c. Induction Welding Large Component Parts
In spite of the advances made by Boeing with regard to fixed coil thermoplastic welding workcells, such presently available fixed coil workcells are generally unsuited to accommodating large sections of skin panels and spars. The dies and other tooling that would be needed to accommodate wing skins and spars, as well as other large aerospace structures having lengths on the order of ten feet or longer, would make such tooling expensive and difficult to manage.
It is difficult with presently available fixed coil thermoplastic welding workcells to obtain highly accurate, localized control over the heat applied (via the susceptor) to small areas of the components being welded (i.e., uniform heating to the desired temperature). As described earlier, long sections of wing skins and spars are usually not perfectly flat, and usually suffer from minor surface imperfections or irregularities. With present fixed coil systems, when such components are brought into contact with the susceptor, the heating of the surfaces of the components being welded can vary because of these surface imperfections. At those small areas of the components where good surface contact is not made with the susceptor, insufficient heating of the component may result. To ensure contact, profiling of the faying surface is possible but is an expensive process that is unsuited to production at appreciable rates. This limitation also makes it quite difficult for fixed coil systems to thoroughly bond components having varying thicknesses along their lengths.
Moving coil systems currently available also have trouble providing highly precise, localized temperature control over large components being welded. With moving coil systems, it is generally even more difficult to control precisely the temperature of the weld zone sufficiently to compensate for surface imperfections. The coil is constantly moving (albeit slowly), while heating small sections of the components, one at a time. In effect, small sections of the components being welded are heated up and begin cooling again, as the coil continues to move, such that it is difficult to control the coil so that it spends sufficient time at any one spot to allow the temperature to be precisely regulated. It is also difficult to maintain the desired pressure along the weld line while using a moving coil system. Accordingly, existing moving coil processes are even less suited than fixed coil process to allow for using temperature feedback control devices to monitor and provide precise adjustments to the coil to alter the temperature of the weld zone slightly, as needed, to provide precise temperature control over the entire area of the components being welded.